Re: PATCHES: start adding Mercurial backend (HgOutputOption)

[from my patches]
>     txn = self.repo.transaction()
>     try:
>       node = self.repo.commitctx(mctx)
>       txn.close()
>     finally:
>       del txn
>     return node

[Michael inquires]
> I don't understand the point of the try...finally here.  "del txn" does
> not force txn.__del__() to be run; it only unbinds the local "txn"
> variable

[I plead ignorance]
> I'll ask on mercurial-devel about this idiom.

Got the answer.  Without the "finally: del txn", an exception thrown
from the "try" clause creates a traceback object that retains
references to every stack frame.  That includes references to all
local variables in each stack frame.  So in that case, the txn
destructor is invoked much later and far away from here.  The "del
txn" removes it from the current stack frame, ensuring that it's not
kept around in case of an exception.

The full thread is here:
http://www.selenic.com/pipermail/mercurial-devel/2009-October/015771.html
.

Greg

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